While the concept is sound, I do hope that you will allow users of your service to 'detach' at some point to make their sites into something more complex. Working around Website as a Service limitations is a tremendously tedious haul. I have worked with a Shopify project for a few months now and having to either write or pay for (via an app) functionality that has existed on any self hosted e-commerce for years is really annoying. Not being able to modify backend code impedes development speed as you try to hack around the backend's deficiencies with Liquid. I didn't want this post to be this long but I just wanted to give my experience on this kind of subject and I hope you take appropiate measures to save your customers from dealing with similar issues.